The present invention relates to a sewing machine with trimming knives, in which an operational knife having a hook-shaped tip is repeatedly projected from a needle plate and comes into slidably contact with a stationary knife disposed at a corresponding position to thereby cut an edge hem of a fabric being fed to needle drop points in a state where the edge hem is folded to the back side for a predetermined width.
For example, when a tube portion for inserting an elastic tape is formed around the waist portion of sweatpants, trousers for an infant, and the like, the edge hem of a target fabric is folded to the back side for a predetermined width, and in such a state, the fabric is fed to needle drop points and hemmed to the surface side of the fabric.
In order to perform the hemming with high efficiency, a sewing machine with trimming knives is used. In the sewing machine, an operational knife having a hook-shaped tip is disposed on one side of the front portion of a needle plate and is moved up-and-down so as to repeatedly project from the needle plate as it is driven by the machine spindle to thereby slidably come into contact with a stationary knife disposed in a corresponding position. The edge hem of the fabric fed to the needle drop points is captured by the tip of the operational knife, the operational knife trims the fabric with the stationary knife while letting a predetermined folding width remain and the resultant fabric feed to the needle drop points.
A sewing machine with trimming knives of this kind, however, has an inconvenience such that the projecting operation of the operational knife in the front position of the needle plate disturbs the feeding of the fabric during regular sewing which is not including the hemming. In order to solve the inconvenience, for example, Japanese Unexamined Patent Application No. 10-287 discloses a sewing machine with trimming knives in which a portion for connecting/disconnecting the transmitting power is provided halfway in a transmitting system between the machine spindle and the operational knife, and the up-and-down movement of the operational knife is stopped by cutting the power transmission to the operational knife.
In the power transmission system to the operational knife, the proximal portion of the operational knife is attached to the knife shaft rotatably attached over a sewing bed so that the tip of the operational knife is moved up-and-down by the oscillation of the knife shaft on it axis. The knife shaft is connected via a link member and a transmitting lever to a oscillating shaft which is in parallel to the knife shaft, the oscillating shaft is connected to the machine spindle via a known eccentric mechanism, and the oscillation of the oscillating shaft according to the rotation of the machine spindle is transmitted to the knife shaft via the link member and the transmitting lever, thereby oscillating the knife shaft.
The connecting/disconnecting portion disclosed in Japanese Unexamined Patent Application No. 10-287 has a construction such the transmitting lever for connecting the knife shaft is divided into two parts in the longitudinal direction, and the two parts are connected/disconnected by insertion/extraction of a coupling pin attached to an output end of an air cylinder. When the coupling pin is inserted, the operational knife is moved up-and-down by the power transmitted from the integrated transmitting lever. When the coupling pin is extracted, the vertical movement of the operational knife is stopped due to the disconnection of the parts of the transmitting lever.
In the connecting/disconnecting portion, the disconnection of the parts of the transmitting lever is realized by a coupling portion which is bent by an upward movement of the operational knife. The connecting/disconnecting portion is constructed so that the power transmission at the time of extraction of the coupling pin is interrupted after the transmitting lever swings in the direction opposite to the bending direction, that is, after the operational knife is moved to the lowermost position where the operational knife gets away below the needle plate, thereby preventing the operational knife from disturbing the feeding of fabric to the needle drop points.
In the connecting/disconnecting portion disclosed in the publication 10-287, a special transmitting lever having a bendable portion in the middle is used. This kind of the transmitting lever has problems with complicated mechanisms and difficulty of construction strong enough to sustain with high-speed sewing. Further, the prior art also has a problem with difficulty of assembling an air cylinder for connecting/disconnecting the coupling pin near an inherent disposing space of the transmitting system below the needle plate.
Japanese Examined Utility Model Publication No. 61-24226 and the like disclose a sewing machine with trimming knives having a stroke adjusting mechanism capable of adjusting a vertical stroke of the operational knife including a zero stroke. According to the sewing machine, when performing a sewing other than hemming, by adjusting the vertical stroke to zero, the projection of the operational knife can be prevented without disturbing the feeding of fabric.
The stroke adjusting mechanism of this kind is, however, relatively large in size because it is constructed by combining a plurality of link mechanisms and thus it requires many components.